1. Field of the Invention
The invention relates to a method of preparing polyamides by anionic polymerization and copolymerization of cyclic lactams.
2. Description of the Prior Art
The anionic polymerization of lactams, catalyzed by strong bases alone, has a satisfactory velocity only at comparatively high temperatures, e.g. at 250.degree. C., i.e. above the melting point of the resulting polyamide. The method cannot be therefore used for polymerization casting. A substantial increase of the reaction velocity takes place only if co-catalysts are added to the monomer-catalyst mixture, i.e. to a mixture of the respective lactam with its alkali metal salt or with an alkali metal hydride. Such co-catalysts, capable of forming centers of growth, are described e.g. in the U.S. Pat. No. 3,017,391.
Many effective co-catalysts of the anionic lactam polymerization have been reported, e.g. diacyl amines, isocyanates and others. Isocyanates are broadly used because of their high activity and easy accessibility. Particularly polyfunctional isocyanates are often used for polymerization whereby very tough polyamide articles such as large gears, bearings and similar are manufactured.
The use of isocyanates has, however, the disadvantage of high toxicity of these compounds, particularly their ability to provoke the growth of cancerous tumors. Moreover, their activity is sometimes too high even at comparatively low temperatures of about 150.degree. C. at which the casting is usually carried out. This is particularly disadvantageous in the manufacture of large castings where the adiabatic course of polymerization is essential. The too high polymerization velocity causes rapid increase of viscosity so that gas bubbles may be trapped in the casting. In the case of lactams polymerizing at a comparatively high rate and yielding polyamides with a comparatively low melting point, the interior of a very large casting may be partly melted, causing contraction cavities. Moreover, in case of rapidly polymerizing lactams such as caprylolactam or laurolactam, the pot-life of the initiated monomer mixture is too short and large castings cannot be made from a singly monomer batch.
The use of low molecular weight nylon-1 polymer is taught in U.S. Pat. No. 3,681,473, to Chow. However, it has to be stressed, that linear oligomers and polymers of isocyanates (nylon 1) are thermally unstable. At temperatures at which lactams are usually polymerized (i.e. 170.degree.-240.degree. C.) the degradation of poly(phenylisocyanate) amounts 76-87%. The main product of thermal decomposition is the highly toxic phenylisocyanate (see Iwakura et al. J. Polymer Sci. A-1,6,2611 (1968)). On the other hand, the six-membered cyclic trimers of isocyanates are thermally very stable and there is no danger of poisoning when solutions of the cyclic trimer in molten lactams are handled during the polymerization. It would generally appear that cyclic trimer compounds having such a high stability inherent to six member rings would be inactive as activators of anionic polymerization.